The present invention relates to an electromagnetic lens for focusing a charged-particle beam such as an electron beam and, more particularly, to improvements in a conical electromagnetic lens.
U.S. Pat. No. 3,707,628 discloses a conical electromagnetic lens of the construction shown in FIG. 1. This lens comprises a conical bobbin 10, a coil 11 wound around the bobbin, and a shroud 12 surrounding the outside of the coil. A beam of charged particles passes through the bobbin 10 along its axis Z. Where this lens is used as the objective lens of a scanning electron microscope, a specimen 13 is placed below the lens. The bobbin 10 is made from a nonmagnetic substance, while the shroud 12 is made from a ferromagnetic substance.
In the conical lens of this structure, a space for installing a detector acting to detect secondary electrons, reflected electrons, or X-rays emitted from the specimen is created close to the lens and, therefore, it is easy to detect the above-described secondary electrons, and so on. Even if the specimen 13 is tilted at the maximum angle, the specimen can be brought close to the principal plane of the lens, thus enabling high-resolution observation. Consequently, this lens is adapted for use in a scanning electron microscope where a flat specimen having a large area, such as a silicon wafer, is tilted for observation.
The coil 11 described above is composed of a conductor wire that is wound on the bobbin with a uniform radial thickness d. That is, the number of turns per unit length along the axis Z of the coil is constant. The lens having this coil 11 shows an axial magnetic field distribution as shown in FIG. 2(a), where the broken line shows the orbit of an electron beam incident on the lens parallel to the axis Z.
In this conical lens, if the half conical angle .alpha. is made small to permit the tilt angle .theta. of the specimen to be made larger, e.g., larger than 60.degree., then it is impossible to reduce the distance Z.sub.o between the position of the principal plane of the lens and the specimen below a certain value. This, in turn, makes it impossible to decrease the spherical aberration coefficient C.sub.s and the chromatic aberration coefficient C.sub.c of the lens below certain values.
These problems are described now in detail. In order to investigate the spherical aberration coefficient C.sub.s of the electromagnetic lens, the present inventors calculated the relation of the spherical aberration coefficient C.sub.s of the lens to the half-value width D of the axial magnetic field distribution, using the distance Z.sub.o between the principal plane of the lens and the focal point, or the position of the specimen, as a parameter. FIG. 3 is a graph diagrammatically showing the results of the calculation. It can be seen from FIG. 3 that (1) the distance Z.sub.o should be made as small as possible to reduce the spherical aberration coefficient C.sub.s and that (2), where the distance Z.sub.o assumes a certain value, the half-value width D should be so selected as to minimize the value of the coefficient C.sub.s.
With respect to the requirement (1), the chromatic aberration coefficient C.sub.c corresponds to the distance Z.sub.o and so reducing the distance Z.sub.o is also effective in reducing the chromatic aberration coefficient C.sub.c With respect to requirement (2), if the distance Z.sub.o is made small, then it is necessary to reduce the half-value width D, as can be seen from FIG. 3.
We now discuss the conical lens shown in FIG. taking account of these facts. Since the position at which the axial magnetic field strength assumes its maximum value is located at a relatively high position, it is difficult to reduce the distance Z.sub.o. Therefore, a limit is set to reduction in the coefficient C.sub.c. In order to adjust the half-value width D, it is desired to make the distribution of the axial magnetic field adjustable, but this is achieved only by adjusting the length of the coil, or the dimension taken along the Z axis. Thus, there exists only a small degree of freedom. Furthermore, it is difficult to reduce the width D itself below a certain value.